An antigen shared by numerous species of a bacterial family may have biologic and clinical significance because: (1) it may be used as single indicator for the quantitation of the immune response; (2) it may serve as immunogen for immunization against infection, and (3) it may function as a pathogenetic factor in immunologic disease. The present study is concerned with several common antigens of enterobacteriaceae, the common enterobacterial antigen, the R core lipopolysaccharide, and the lipoprotein. The cellular immune response will be studied in several animal species. Rabbits, guinea pigs, and selected strains of mice will be immunized with each antigen, either intravenously, intraperitoneally, or subcutaneously. Additional animals will be injected also with complete Freud's adjuvant. Depending upon the animal species used, one or more of the following tests will be used: (a) skin test; (b) H3-thymidine incorporation into lymphocytes (blastogenesis); (c) macrophage aggregation; (d) macrophage disappearance reaction; and (e) chemotaxis of macrophage and polymorphonuclear cells. The findings will be correlated with the humoral antibody response. To determine whether immunization results in immunologic injury, the animals will be studied histologically. The relationship between molecular size of lipopolysaccharides and biologic activity will be determined by the study of uniform salt forms. These preparations will be regarding: (1) cell affinity, (2) mitogenic effects, and (3) immunogenicity, and collaboration with Dr. C. Galanos of the Max-Planck Institut fur Immunbiologie, Freiburg, Germany.